XIENG KHOUANG, Laos (Reuters) - Yianyang Bounxieng remembers playing with bombs as a child growing up in Xieng Khouang, a verdant and mountainous province of Laos, and the area most heavily bombed by U.S. Air Force planes during the war in neighboring Vietnam.

His grandmother resented foreigners because of the bombings.

"When I took tourists to visit her she was angry and would say: 'Why did you bring them here? They destroyed everything'.

Now 28-year-old Yianyang works as a support officer for Mines Advisory Group (MAG), which helps to find and destroy unexploded ordnance (UXO).

Addressing the legacy of war in Laos will be a focus of U.S. President Barack Obama's trip this week to the country's capital, Vientiane, for a meeting with Association of Southeast Asian Nations (ASEAN) leaders and an East Asia Summit.

Obama, who will become the first sitting president of the United States to visit Laos, is expected to announce more funding to help clear leftover bombs and conduct Laos' first national survey on unexploded ordnance.

"The big focus of the UXO programme over the next few years will be to conduct this comprehensive national survey on cluster munitions," Balasubramaniam Murali, deputy resident representative to the United Nations Development Programme in Laos, told Reuters.

From 1964 to 1973, U.S. warplanes dropped more than 270 million cluster munitions on Laos, one-third of which did not explode, according to the Lao National Regulatory Authority for UXO (NRA).

The bombings were part of a CIA-run, secret operation aimed at destroying the North Vietnamese supply routes along the Ho Chi Minh trail and wiping out its communist allies.

They also left a trail of devastation in Laos, which U.S. planes used as a dumping ground for bombs when their original target was unavailable and planes couldn't land with explosives.

U.S. bombs are still killing in Laos, and Xieng Khouang could be earning more if it was safe for visitors to trek on its rolling hills.

Across the country, over 20,000 people have been killed or injured by bombs since the war, many of them children.

According to Legacies of War, an organization focused on addressing the impact of bombs dropped on Laos, $25 million a year will need to be spent over the next decade to prevent further casualties.

CHILD'S PLAY

Beeping metal detectors pierce the tranquillity of Phaxay district in Xieng Khouang, where local technicians scour a hillside for unexploded bombs. They find a BLU-26 cluster bomb, the size of a tennis ball, and mark it for demolition. Another 106 BLU-26 bombs were found on site over the past two months.

The Lao government says a national survey will take five years to finish but Phouttasone Nounthabout, 22, the team's deputy leader, says that's probably impossible.

"There is so much land to clear," she told Reuters.

While overall casualties have come down – in 2008 recorded casualties were just over 300 compared to 42 in 2015 – the percentage of children hurt or killed has gone up.

"Children are curious and tend to play with the bombs or try to prise it open," said Murali.

Landlocked Laos remains largely agricultural with around 80 percent of the population reliant on agriculture. Some land is simply too dangerous to farm.

Now 40 years old, Soud was 10 when, out farming with his family, his spade hit a bomb triggering an explosion that blinded and maimed him.

His mother, Thongsy, now 75, remembers the day vividly.

"I heard an explosion and then I saw my child lying there. The villagers helped carry him to the nearest hospital by foot. They had to cut off his hand. I was crying," she said.

Pomee Kaewpimpa, 59, was a young boy when he saw bombs dropped on Napia village in Xieng Khouang up to three times a day.

Now a village elder, he wants Washington to take responsibility.

"Until every bomb is removed from the ground our children will be at risk," he said. "I want to know whether those Americans who pierced our land with bombs, are they sorry?"